For hydrogen production in vehicle dimethyl ether (DME) steam reforming reaction system, the thermodynamic efficiency of reforming reaction system was analyzed. The change of chemical equilibrium constant in main reactions with temperature was confirmed. The effects of steam-to-DME ratio, temperature and pressure on equilibrium were studied. The results indicated that DME reformed gas increased low heat value by 15.45％. DME had a high conversion when the temperature was higher than 473K and steam-to-DME ratio was more than 2, and the equilibrium concentration was close to zero. The main components of reformed gas were H2, CO and CO2. The content of H2 showed the most, and the largest concentration was 72%. With high temperature and low steam-to-DME, carbon in DME would be converted to CO. With low temperature and high steam-to-DME, carbon in DME would be converted to CO2. Pressure had little effect on the reaction equilibrium concentration.